This invention relates to the field of turbine engines and, more specifically, to a method and apparatus for prolonging the life of turbine blades by reducing thermo-mechanical stresses in thermally cycled engine parts.
Turbine engines are commonly used in the generation of electrical power. The useful work obtained from a turbine engine is in the form of rotational shaft power. This shaft power can be fed directly into an electric power generator to produce electricity. Typically, a power company will run large turbine engine powered generators to satisfy the base load demand then either operate smaller output generators or purchase power from other sources during periods of peak demands. This strategy worked well when the energy producing community had high excess capacity. Ever increasing energy demands have shrunk excess capacity, making the peak power requirements larger than ever. The demand can get high enough to require the operation of additional large turbine powered generators. Of the large turbine powered generators, the only type of turbine that can provide power in a timely fashion are gas turbines. The result is that large gas turbine powered generators are being cycled frequently to full power, in an effort to satisfy peak demand. These large gas turbines are not designed to be operated in this fashion. They were designed to be turned on and left on for long periods of time to satisfy the base load demand. Not operating the engine in this fashion constitutes a change in the duty cycle of the engine.
The expected lifetime of the components in a gas turbine engine are is predicted analytically during the design phase. This process is referred to as lifting analysis. A key element of the lifting analysis is the duty cycle. Deviation from the designed duty cycle can have a drastic effect on the life of gas turbine components. As expected, the gas turbines used for peak demand power generation have a high rate of component failure. This is very costly in both parts and machine down time. To create a solution to the problem that does not involve a new engine designed specifically for the task, the nature of the problem must be understood.
As stated earlier, the nature of the current energy market has resulted in the operation of gas turbines outside their designed duty cycle. The present duty cycle now includes a much higher frequency of power level changes. These power level changes are the root of the problem. For example, when an engine is operating at idle, the rotational speed and gas temperatures inside the engine are low. As the engine transitions to full power, both rotational speed and the gas temperature increase dramatically. Increasing rotational speed increases the mechanical stress in the part. When a relatively cool part is immersed in a much higher temperature fluid, an additional stress is caused, called thermal stress. These two types of stresses together can produce an overall stress level high enough to permanently weaken a part. Repeating this condition eventually causes the part to fail. The present invention reduces thermal stresses by thermally conditioning the components before the temperature change occurs. For example, if a temperature increase is to occur, a part might be preheated in a controlled fashion. This would result in a lowering of the thermal stresses, thus extending the life of the part.
The prior art is replete with teachings of cooling gas turbine parts by bleeding off compressor air and routing it to the parts to be cooled via a secondary airflow system. In these systems, the compressor establishes the pressure, temperature and flow rate of the coolant. The compressor does not operate at conditions that provide optimal pressure, temperature and flow rate at all times.
Representative of such systems are: U.S. Pat. No. 4,190,398 which discloses a system to cool turbine blades using compressor air flowing through a heat exchanger operating as a thermosiphon to cool engine parts.
U.S. Pat. No. 4,815,928 which discloses a two position, off-on, valve and controller to regulate the air flow rate from the compressor to the engine parts.
U.S. Pat. No. 3,949,549 which discloses a heat exchanger in the coolant line and a valve to control flow rate of the coolant.
None of these prior art devices teach the modulation of the coolant fluid temperature and pressure and flow or any combination thereof to prolong engine life.
Disclosed is a method and apparatus for controlling thermal stress in a turbine engine which includes the steps and structure for heating or cooling the thermally cycled engine parts for a certain amount of time at a certain rate before the operating point of the engine is changed.
Accordingly, it is an objective of the instant invention to teach a method of altering the duty cycle of any gas turbine in such a way so as to decrease thermal stresses thus increasing the usable lifetime of thermally cycled parts.
It is a further objective of the instant invention to teach the modulation of temperature and pressure and flow of the coolant fluid to the thermally cycled parts of the engine.
It is yet another objective of the instant invention to teach the use of an engine controller which will compare status and projected change and initiate heating or cooling of the coolant air flow and determine time and coolant flow rate required to adjust the engine to a desired operating point. The controller may perform the functions automatically or inform an operator for manual execution.
It is a still further objective of the invention to teach the use of electric resistance heating or thermoelectric devices in the engine to heat or cool the thermally cycled parts. These electrical elements may be used independently or in conjunction with the coolant fluid.
Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.